Apart from some early reports on the elusive and extremely labile tellurium nitride compounds TeN, Te3N4, and Te4N4, and the recently discovered [Te6N8(TeCl4)4],1 the only structurally characterized binary Te–N species is the salt [Te(N3)3][SbF6].2 The triazidotelluronium cation was obtained as an unexpected product during an attempted preparation of [Te2N]+. The chemistry of tellurium azides was initiated by Wiberg in the 1970s.3 Various examples of homoleptic azido transition and main-group metalates and nonmetalates are reported,4 but none of the chalcogen group. The groups of Wiberg and Passmore have indicated the possible existence of a highly explosive Te(N3)4 and warned of their potential danger. In pursuit of our recent efforts to explore the chemistry of covalent and ionic tellurium azides,5, 6 we describe here the synthesis, isolation, and properties of Te(N3)4 (1),7 [Me4N][Te(N3)5] (2 a) and [pyH][Te(N3)5] (2 b), the latter containing a rare pentacoordinated polyazido anion, the only other example being the [Fe(N3)5]2− ion.8
As is known from the syntheses of TeCl3(N3) and TeCl2(N3)2,3 and confirmed by our own studies, treatment of TeCl4 with excess Me3SiN3 does not result in the substitution of all chlorine atoms. All successful preparations for R2Te(N3)2 and RTe(N3)3 (R=alkyl, aryl, trifluoromethyl and perfluoroaryl) proceed via the corresponding tellurium di- or trifluorides R2TeF2 and RTeF3.5, 6a Thus, for the synthesis of Te(N3)4 (1) and [Te(N3)5]− (2), the fluorinated species TeF4 and [Me4N][TeF5] were preferred as starting materials. Tellurium tetrafluoride rapidly reacts with Me3SiN3 in CFCl3 (0 °C) suspension to form a yellowish precipitate of 1 that is soluble in DMSO [Eq. (1)((1))]. On one occasion, the solid exploded violently when obtained from a suspension in CH2Cl2. Solutions of 1 in [D6]DMSO exhibit a very broad 125Te NMR resonance at δ=1380 ppm, deshielded compared to that of TeF4 (δ=1195 ppm). The resonance for 1 is identical to that detected in a mixture of the dismutation products of C6F5Te(N3)3, Te(N3)4, and (C6F5)2Te(N3)2.6a Due to our experience with the unpredictable explosiveness of neat 1, vibrational spectra, mass spectra, and elemental analysis were omitted.9
In a similar fashion, [Me4N][TeF5] reacts with Me3SiN3 in CH2Cl2 to form a yellow solution of [Me4N][Te(N3)5] (2 a) [Eq. (2)((2))], from which yellow crystals can be grown at below −20 °C. In some cases, an extremely sensitive yellow oil also separates, which in one case exploded when the cold mixture was stirred. For 2 a, a sharp 125Te NMR resonance was observed at δ=1258 ppm (CD2Cl2), again downfield from that of [TeF5]−. The 125Te NMR resonance of [TeF5]− was observed at 25 °C in CH2Cl2 solution in the presence of [Ph4P]+ ions as a doublet of quintets at δ=1161 ppm (1J(125Te-Fapical)=2918 Hz; 1J(125Te-Fbasal)=1386 Hz). The Raman spectrum of 2 a shows bands at 2105/2055 [νas(N3)] and 409/347 [ν(TeN)] cm−1, which are detected in the typical regions for tellurium azides.5, 6 A reaction between [Me4N]2[TeCl6] and Me3SiN3 did not occur, and the [TeF6]2− ion remains still unknown.10
In an attempt to stabilize the tetraazide 1, and to gain products more easily characterizable, an effort was made to react a pyridine⋅TeF4 adduct with azide. Such TeF4 adducts are reported, but except for elemental analysis, no analytical information is available.11 In order to confirm that compounds of the proposed form [L⋅TeF3][TeF5] (L=Me3N, pyridine etc.) were prepared,12 further analytical information was desirable. In the course of the preparation of such a proposed pyridine⋅TeF4 adduct, the results of the crystal-structure determination unexpectedly revealed a complex mixture of pyridinium pentafluorotellurate(IV) and dimeric units of TeF4 solvated by pyridine.13
Since crystals of 2 a grown from the reaction solution tend to deliquesce very rapidly, crystallization was attempted from solutions of the mixture of pyridine with TeF4 and Me3SiN3. Although the crystals obtained after several weeks were shown to be the pyridinium salt [pyH][Te(N3)5] (2 b), probably formed by reaction of TeF4 with fluoride in the employed glass vessel to give [TeF5]− ions, the resulting pentaazidotellurate(IV) anion was unaffected. The pyridinium salt 2 b crystallizes in the triclinic space group P, for the anion a distorted Ψ-octahedral TeEN5 coordination is found (Figure 1). The [Te(N3)5]− ion represents the first structurally characterized anionic tellurium azide.7 Similar to neutral organotellurium(IV) azides,5, 6a secondary Te⋅⋅⋅N interactions below the sum of the van der Waals radii (3.61 Å)14 create network structures, which result in the octacoordinated tellurium atoms in 2 b (Figure 2). The TeN bond lengths vary from 2.075(2) Å (Te-N7) for the apical azide group, to between 2.175(2) and 2.256(2) Å for the basal azide groups. The apical N3 unit has the shortest NαNβ/NβNγ bond lengths and the smallest N-N-N angle. The N-Te-N bond angles range between 74.53(7) and 165.99(8)°, and the N-N-N bond angles are 175.8(2)–177.9(3)°. The rather irregular square-pyramidal structure likely results from electrostatic repulsions of the differently polarized Nα and Nβ atoms of the azide groups, both between each other, and with the lone pair of the TeIV center. Two sets of virtually identical TeN distances for two geminal basal azide groups are present with four different orientations towards the apical position (Figure 1).
The electronic structure of the [Te(N3)5]− ion was calculated using different ab initio and density functional methods and basis sets (Table 1). All geometries have been fully optimized at the level chosen and led to distorted C1-symmetric minimum structures in good agreement with the experimental crystal-structure determination. Best results regarding bond lengths and vibrational frequencies were obtained by using the B3LYP/SDD combination. However, this finding may be due to accidental compensation of pseudopotential and basis deficiencies, which may suggest a higher degree of accuracy for this cheaper method.
|Method experiment||E [Hartree]||zpe [kcal mol−1]||d(Te-Napical) [Å]||d(Te-Nbasal) [Å]||as(N3) [cm−1][a]|
After the structural characterization of binary Te azide species of the type [Te(N3)3]+2 and [Te(N3)5]− (2 a and b) and the first direct synthesis and NMR spectroscopic characterization of the neutral tellurium azide molecule, Te(N3)4 (1), the race is now on for the isolation of the first selenium azide species. Studies in this regard are currently in progress.15